A semiconductor element such as a switching element or a diode is used in a circuit such as a switching power supply or an inverter. These semiconductor elements are required to have a high breakdown voltage and a low on-resistance. A relation between the breakdown voltage and the on-resistance is a trade-off relation depending on an element material.
Due to the progress of the technological development so far, a semiconductor element having a low on-resistance near a limit of silicon as a main element material has been realized. In order to further improve the breakdown voltage or further reduce the on-resistance, it is necessary to change the element material.
A GaN-based semiconductor such as gallium nitride (GaN) or aluminum gallium nitride (AlGaN) has a larger bandgap than silicon. By using a GaN-based semiconductor as a switching element material, the trade-off relationship depending on materials can be improved, and dramatically high breakdown voltage and low on-resistance can be achieved.
However, for example, when a high drain voltage is applied to a switching element using a GaN-based semiconductor, a problem called “current collapse” to increase on-resistance occurs. In order to improve reliability of a switching element, it is necessary to suppress the current collapse.